Electromagnetic actuator magnetically locked into two or more stable positions

ABSTRACT

An electromagnetic actuator has four positions which are stable in the absence of a current and moves rapidly between these positions when acted upon by a current. The actuator has a thin, rotatable magnet (3) with two pairs of magnetic poles transversally magnetised in alternate directions, and a stator member having four pole pieces (9-12, 34-37, 48-51) of developed length P, each with an excitation coil. Two consecutive poles of the thin magnet (3) are spaced by a distance d. The thin magnet (3) is movable in an airgap of width E. wherein the size ratio P:E and P:d is greater than 8. The invention is useful for driving a device such as a valve or a switch.

TITLE OF THE INVENTION BACKGROUND OF THE INVENTION Field of theInvention

The present invention relates to an electromagnetic actuator intended tooperate devices such as valves or switches and having a plurality ofpossible states, controlled by the positioning of a rotary element.

Discussion of Background

The displacement detween two consecutive stable positions constitutes arapid transition achieved by application of a current in at least onepair of coils. The operation of such devices by electric motors usuallynecessitates the use of locking mechanisms to guarantee positioning atrest in one of the sought positions after a rotary displacement, and toprevent untimely displacements in the absence of control signals. Theprior art teaches the technique of actuators provided with a rotordisposed in the air gap of a stator structure formed by a first magneticcircuit provided with four pole shoes, each equipped with an excitingcoil. The rotor has a magnetized thin portion comprising two pairs ofthin poles magnetized transversely in opposite directions. Suchactuators are intended in particular for control of valves or switches.These actuators exhibit the feature of having a broad zone in which theforce is constant. As a result, it is possible to construct actuatorshaving very great reproducibility and very great angular precision underautomatic control.

To achieve positions that are stable in the absence of current, theprior art actuators are usually provided with a ratchet wheel to lockthe position of the actuator after a displacement. The mechanical partsemployed for locking are a source of noise and wear. In addition, theylead to higher ts for manufacture and assembly of such actuators.

SUMMARY OF THE INVENTION

The object of the present invention is to construct an actuator providedwith at least two stable positions, with a strong locking torque with orwithout current, not necessitating mechanical locking parts.

To this end the present invention relates to an electromagnetic actuatorhaving four positions that are stable in the absence of current, foroperation of a device such as a valve or switch, the actuator beingprovided with a thin rotationally movable magnet, having two pairs ofmagnetic poles magnetized transversely in alternate directions, and astator structure provided with four pole shoes of developed length P,each equipped with an exciting coil, two consecutive poles of the thinmagnet being separated by a distance d. The thin magnet is movable in anair gap of dimension E, the dimensional ratios P/E and P/d being largerthan 8.

Such an actuator has a locking torque of almost 30% of the motor torquewhen the diametral plane separating the two magnets is in one of theplanes passing between two pole shoes.

Such an actuator can be constructed in two variants: a first variant inwhich the movable portion has the shape of a disk, and a second variantin which the movable portion has the shape of a tube.

According to the first variant, the movable portion comprises a yoke ofcircular cross section integral with a thin magnet in the shape of adisk, having two complementary sectors extending for about 180° andmagnetized in two opposite axial directions, the pble shoes of thestator portion disposed facing the movable magnet having an annularshape extending for about 90°.

Advantageously, the semi-annular surface of the pole shoes of the statorportion is extended by a foot of smaller cross section, the excitingcoil encircling the said foot.

Preferably the shoes are integral with a disk-shaped magnetic-fluxclosing part.

According to one particular embodiment, the stator structure comprises afirst ferromagnetic part having two pole shoes and a secondferromagnetic part having two pole shoes, the two stator parts beingdisposed on both sides of the thin disk-shaped magnet, the planespassing through the top surface of the pole shoes being spaced apart bya distance d, and the two stator parts being offset by 90°.

According to a second constructional variant, the stator portion isformed by an inner part of cylindrical shape of ferromagnetic material,having two diametrically opposite lateral notches for positioning of anelectric coil, and by a peripheral part of tubular shape having twolongitudinal inner grooves for positioning of a second electric coil,the median planes of the two coils being perpendicular.

According to a first embodiment, the movable portion comprises a yoke ofcylindrical shape and a thin cylinder-shaped magnet having twocomplementary tile-shaped sectors extending for about 180° andmagnetized in two opposite radial directions.

Advantageously, the stator portion is formed by an inner part ofcylindrical shape of a ferromagnetic material, having two diametricallyopposite lateral notches for positioning of an electric coil, and by aperipheral part of tubular shape having two longitudinal inner groovesfor positioning of a second electric coil, the median planes of the twocoils being perpendicular.

According to a second embodiment, the stator structure is formed by aninner part of cylindrical shape of a ferromagnetic material, and by aperipheral part of tubular shape having four longitudinal inner groovesfor positioning of four electric coils, the median planes of the twocoils being angularly offset by 90°.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood in the descriptionhereinafter, provided with reference to the drawings, wherein:

FIG. 1 represents an exploded view of an actuator according to the firstvariant,

FIG. 2 represents an overhead view of the pole shoes,

FIG. 3 represents the curve of actuator torque versus position,

FIG. 4 represents a median sectional view of the actuator,

FIG. 5 represents an exploded view of a second embodiment,

FIG. 6 represents an exploded view of a third embodiment,

FIG. 7 represents a transversal sectional view of a secondconstructional variant of the actuator,

FIG. 8 represents a transversal sectional view of another embodiment ofthe actuator.

FIG. 1 represents an exploded view of an actuator according to the firstconstructional variant.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The actuator comprises a movable portion (1) and a fixed portion (2).

The movable portion (1) is provided with a thin magnet (3) formed fromtwo pairs of poles (4, 5) magnetized axially in opposite directions,each extending for about 180°. The transitions (6, 7) between two pairsof poles (4, 5) are smaller than 1°. The magnet (3) is constructed byjoining two half magnets or preferably by magnetizing two sectors of athin disk of a material such as ferronickel or any other material usedfor construction of permanent magnets.

The movable portion (1) is also provided with a yoke (8) offerromagnetic material. The magnet (3) is attached close to this yoke(8).

The fixed portion comprises four pole shoes (9 to 12) formed by annularsectors extending for almost 90° and attached by feet (14 to 17) to aflux-closing plate (13). Electric coils (18 to 21) encircle the feet (14to 17) to excite the pole shoes (9 to 12). Two pole shoes are separatedby a free space (22 to 25), the length of which, measured along the meanperimeter, is d.

The actuator functions as follows:

In rest position No. 1, the first pair of magnet poles (4) having theSOUTH pole up is aligned with the two shoes (10, 11), the other pair ofmagnet poles (5) being aligned with the other two shoes (9, 12). Thetransitions (6, 7) are aligned respectively with the diametricallyopposite spaces (25), (23).

This first rest position is stable in the absence of current. It isnecessary to apply a significant torque to displace the movable portion.

In what follows, it will be considered that the flow of a positive (orrespectively negative) current in one of the coils tends to cause theNorth (or respectively SOUTH) pole of the magnet to be displaced untilthe potential of the magnetic poles is added to that of the magnet.

Departure from rest position No. 1 is achieved by energizing the twoconsecutive coils (20, 21) positively and the two consecutive coils (18,19) negatively. The movable portion is then displaced by 90°, ultimatelyoccupying a rest position No. 2. This position corresponds to alignmentof the first pair of magnet poles (4) having the SOUTH pole up with thetwo shoes (11, 12), the other pair of magnet poles (5) being alignedwith the other two shoes (9, 10). The transitions (6, 7) are alignedrespectively with the diametrically opposite spaces (22), (24).

To return from rest position No. 2 to rest position No. 1, it isnecessary to energize the two coils (19, 20) positively and to energizethe two coils (18, 21) negatively.

If the actuator is intended to cause an element to pivot between twopositions offset by 90°, as for a valve control, the two diametricallyopposite coils (18, 20) are always energized in the same manner.Pivoting from one of the positions to the other position is achieved byinverting the polarity of energization of the other two diametricallyopposite coils (19, 21).

It is also possible, by combined energization specific to the purpose,to displace the movable element (1) into the other rest positions thatare offset by 90° relative to the rest positions No. 1 and No. 2.

FIG. 2 represents an overhead view of the pole shoes.

The dimensional characteristics are determined by measurement along themean perimeter (26). The mean perimeter (26) is the circle whose radiuscorresponds to the mean between the radius R1 of the inner edge of theshoes (9 to 12) and the radius R2 of the outer edge of the shoes (9 to12).

In the following,

P designates the width of the pole shoes (9 to 12),

E designates the air gap between the top of the pole shoes (9 to 12) andthe bottom surface of the movable yoke (see FIG. 4),

d designates the distance between two consecutive pole shoes (9 to 12).

One of the characteristics necessary to achieve the technical effectconstituting the object of the invention, or in other words optimizationof the amplitude and stiffness of the currentless locking torque, liesin the choices of dimensional ratios.

A large value, typically greater than 8, will be chosen for the ratioP/E. A value close to P/8 will be chosen for the distance d.

In this case, the actuator exhibits maximum currentless locking torque(typically 30% of the nominal torque with current) over the four stablepositions, and maximum stiffness of this currentless torque law(typically 10 mNm/degrees), while retaining a starting torque sufficientfor acceleration of the rotor and an end-of-travel stiffness withcurrent on sufficient to brake the rotor. FIG. 3 represents a typicalexample of the end of torque with or without current as a function ofposition.

FIG. 4 represents a median sectional view of the actuator through theplane A-A'.

The movable portion (1) is integral with a shaft (27) passing throughthe stator portion (2). The air gap E separating the bottom surface (28)of the yoke (2) and the top surface (29) of the pole shoes (9 to 12) isdetermined by means of a thrust ball bearing (30). Positioning of themovable portion (1) and of the stator portion (2) is ensured by themagnetic attraction of the permanent magnets, thus avoiding the use ofadditional mechanical means to ensure immobilization of the shaftrelative to the stator structure (2). In the described example,therefore, the actuator has a single stop acting to limit the axialtravel in the direction of the movable portion (1) toward the statorportion (2), but does not have a stop in the opposite direction.

FIG. 5 represents a second embodiment of the actuator in the "diskmagnet" configuration.

In this embodiment, the disk magnet (3) is integral not with a yoke butonly with a connecting part with a shaft, which is not shown in thisfigure.

The fixed portion is provided with two stator portions (31, 32) disposedon both sides of the magnet (3). Each of the stator portions (31, 32) isprovided with two pole shoes, respectively (34, 35) and (36, 37). Eachof the pole shoes (34 to 37) is excited by a coil (38 to 41). The twostator portions (31, 32) are offset angularly in such a manner thattheir planes of symmetry BB' and CC' form an angle of 90° between them.

In this embodiment, the pole shoes can extend for an angle larger than90°.

In this way, the distance d between the ends of two consecutive shoescan be reduced to 0 or can even have a negative sign. Preferably, thepole shoes (35 to 37)

extend over the largest possible angular aperture, and allow a minimumspace to be present for passage of coils.

FIG. 6 represents another embodiment of an actuator of the disk-magnettype. The two stator portions (31, 32) are symmetrical relative to theplane of the thin magnet (3), and each is provided with four pole shoes(9 to 12). The pole shoes of one of the pole [sic: stator?] portions(32) are excited by electric coils (18 to 21), while the pole shoes ofthe other stator portion (31) can be optionally excited or non-excited.

FIGS. 7 and 8 represent two embodiments of the actuator variant with thetubular magnet.

FIG. 7 represents an embodiment in which the stator portion is providedwith an inner cylindrical part (40) integral with the tubular magnetformed by two pairs of poles (41, 42) in the shape of tiles extendingfor about 180°, each magnetized radially in opposite directions. It isalso possible to provide an inner cylindrical part that is not integralwith the tubular magnet.

The outer stator portion (43) comprises a cylindrical part having fourgrooves (44 to 47), which define between them the four pole shoes (48 to51). Each of the shoes (48 to 51) is encircled by a coil in the shape ofa loop (52 to 55).

For the actuators according to the tubular variant, the dimensions P andd are measured along the internal surface of the pole shoes.

The energization sequences are described hereinbelow with the conventionthat the rotor is represented in FIG. 7 in -45° position.

    ______________________________________                                        Position of                                                                   rotor at end                                                                  of travel              Coil 2    Coil 3                                                                              Coil 4                                 ______________________________________                                         -45°                                                                             +       +          -     -                                         -135°                                                                                    +                                                                                    -         -                                                                                   +                                    +135°                                                                                    -                                                                                    -         +                                                                                   +                                     +45°                                                                                     -                                                                                   +         -                                                                                   +                                    ______________________________________                                    

FIG. 8 represents another embodiment of an actuator with tubularstructure.

The stator portion is provided with an inner part (56) of cylindricalshape having two diametrically opposite grooves (57, 58). A loop-typecoil (59) encircles the inner stator portion (56) and is lodged in thetwo grooves (57, 58). The two grooves (57, 58) define between them thefirst two shoes (60, 61). The second stator portion comprises a tubularpart (62) also having two grooves (63, 64) offset by 90° relative to thetwo grooves (58, 59) of the inner part (56). The other two shoes (66,67) are formed between these two grooves (63, 64). A second electriccoil (65) is lodged in these grooves (63, 64). In the described example,these grooves are formed on the inner surface of the tubular part (62).

It is understood that these examples of coil arrangements are describedby way of example, and in no way do they constitute an exhaustive listof conceivable options.

The present invention is in no way limited to the foregoing embodiment,but to the contrary covers all variants.

I claim:
 1. An electromagnetic actuator having four positions that arestable in the absence of current and in which rapid displacement occursbetween said stable positions under the action of a current, foroperation of a device such as a valve or switch, the actuatorcomprising:a yoke; a rotationally movable magnetic disk, having twopairs of magnetic poles magnetized traversely in alternate directions,and a stator structure provided with four pole shoes each of developedlength P, each equipped with an exciting coil, each two consecutivepoles of said four pole shoes being separated by distance d, themagnetic disk being movable in an air gap of dimension E between saidstator and said yoke, the dimensional ratios P/E and P/d being largerthan 8, wherein the stator structure is configured in such a manner asto ensure magnetic locking in each of said four stable positions.
 2. Anelectromagnetic actuator according to claim 1, characterized in that themovable portion comprises a yoke of circular cross section integral withsaid thin magnet in the shape of a disk, having two complementarysectors extending for about 180° and magnetized in two opposite axialdirections, the pole shoes of the stator portion disposed facing themovable magnet having an annular shape extending for about 90°.
 3. Anelectromagnetic actuator according to claim 1, characterized in that thesemi-annular surface of the pole shoes of the stator portion is extendedby a foot of smaller cross section, the exciting coil encircling thesaid foot.
 4. An electromagnetic actuator according to claim 1,characterized in that the shoes are integral with shapedmagnetic-flux-closing part.
 5. An electromagnetic actuator according toclaim 1, characterized in that a first pair of two opposite coils isenergized by a constant current, and the other portion of opposite coilsis energized by a current whose polarity can be alternated.
 6. Anelectromagnetic actuator according to claim 1, characterized in that thestator structure comprises a first ferromagnetic part having two poleshoes and a second ferromagnetic part having two pole shoes, the twostator parts being disposed on both sides of the thin disk-shaped magnetthe planes passing through the top surface of the pole shoes beingspaced apart by a distance E and the two stator parts being offset by90°.
 7. An electromagnetic actuator according to claim 1, characterizedin that the stator portion is formed by an inner part of cylindricalshape of a ferromagnetic material, having two diametrically oppositelateral notches for positioning of an electric coil, and by a peripheralpart of tubular shape having two longitudinal inner grooves forpositioning of a second electric coil, the median planes of the twocoils being perpendicular.
 8. An electromagnetic actuator according toclaim 1, characterized in that the stator portion is formed by an innerpart of cylindrical shape of a ferromagnetic material, and by aperipheral part of tubular shape having four longitudinal inner groovesfor positioning of four electric coils, the median planes of the twopairs of coils being angularly offset by 90°.
 9. An electromagneicactuator according to claim 2, characterized in that the semi-annularsurface of the pole shoes of the stator portion is extended by a foot ofsmaller cross section, the exciting coil encircling the said foot. 10.An electromagnetic actuator according to claim 2, characterized in thatthe shoes are integral with a disk-shaped magnetic-flux-closing part.11. An electromagnetic actuator according to claim 3, characterized inthat the shoes are integral with a disk-shaped magnetic-flux-closingpart.
 12. An electromagnetic actuator according to claim 2,characterized in that a first pair of two opposite coils is energized bya constant current, and the other portion [sic: pair?] of opposite coilsis energized by a current whose polarity can be alternated.
 13. Anelectromagnetic actuator according to claim 3, characterized in that afirst pair of two opposite coils is energized by a constant current, andthe other portion [sic: pair?] of opposite coils is energized by acurrent whose polarity can be alternated.
 14. An electromagneticactuator according to claim 4, characterized in that a first pair of twoopposite coils is energized by a constant current, and the other portion[sic: pair?] of opposite coils is energized by a current whose polaritycan be alternated.